1. Field of the Invention
The present invention relates to an apparatus for detecting the degree of deterioration of a machine oil, e.g., the engine oil of a vehicle.
2. Description of the Related Art
In a conventional apparatus for detecting deterioration of the engine oil of a vehicle, a metal lead wire is used as an oil deterioration sensor. This metal lead wire is corroded by an acidic substance produced due to deterioration of the oil, and thus the resistance of the wire is increased or the wire is eventually disconnected.
However, in the conventional apparatus, the texture of the metal lead wire is dense, the rate of corrosion by the acidic substance is low, and the wire is corroded only on the order of angstroms. Thus, to precisely detect deterioration of the oil, a very thin metal lead wire must be used. Accordingly, when electrodes are connected directly to two ends of such a very thin metal lead wire, the mechanical strength of the wire cannot be guaranteed. In particular, in a vehicle subjected to continuous external vibration, and in which the pressure of the oil in the oil pan is increased under a high engine load, the metal lead wire may become disconnected even though the oil is not deteriorated, resulting in an erroneous detection signal.
In the conventional apparatus, an iron (Fe) wire is used as the metal lead wire. However, after extensive research, the present inventors have found that an iron wire does not exhibit a linear change in resistance with respect to oil deterioration. That is, according to their research, the present inventors have found that the surface of an iron wire was corroded by acid in the oil immediately after the wire was dipped in the oil, changing the resistance of the wire. However, thereafter, a very thin passive film of a carbonyl group was formed on the surface of the iron wire due to chemical reaction, and the iron wire could no longer be corroded.
Thus, although an apparatus has been proposed wherein a metal lead wire dipped into oil is used as an oil deterioration sensor, an optimum material and structure have not been found, and this has caused a critical problem in practice. The above conventional machine oil deterioration detection apparatus is described in, e.g., Japanese Unexamined Utility Model Publications (Kokai) Nos. 59-91408 and 59-88207.